1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an image forming apparatus capable of forming images on both faces of a recording material.
2. Description of the Related Art
FIG. 10 shows an example of a conventional image forming apparatus in which a full-color image is formed by transferring a plurality of toner images having different colors onto a single recording material in a superposed fashion. This image forming apparatus includes a photosensitive drum (image bearing member) 2 rotatably supported by a body 1 of the apparatus for rotational movement in a direction shown by the arrow. Around the photosensitive drum 2, there are arranged, in order along a rotational direction of the drum, a charge device 3, an optical system 5, a developing means 6, a transfer device 7 and a cleaning device 9.
The optical system 5 comprises an original scanning portion, a color decomposing filter, and a laser beam exposure device for illuminating a color-decomposed light image E or equivalent light image E onto the photosensitive drum 2. By illuminating the light image E for each color onto the photosensitive drum 2 uniformly charged by the charge device 3, an electrostatic latent image is formed on the photosensitive drum 2. The developing means 6 is of rotatable type and includes a rotary member 6b rotated around a central shaft 6a, and four developing devices (i.e. black developing device 6BK, cyan developing device 6C, magenta developing device 6M and yellow developing device 6Y) mounted on the rotary member. A desired developing device is brought to a developing position where the developing device is opposed to the photosensitive drum 2 by rotating the rotary member 6b, so that the electrostatic latent image formed on the photosensitive drum 2 is developed with toner (including resin as a base material) to obtain a toner image.
Then, the toner image on the photosensitive drum 2 is transferred onto a recording material supplied to a transfer position (opposed to the photosensitive drum 2) from a recording material cassette 10 through a sheet path (shown by the dashed line in FIG. 10) by a convey system and the transfer device 7. The transfer device 7 comprises a transfer drum 7a, a transfer corona charger 7b, an absorb corona charger 7c for electrostatically adsorbing the recording material, an adsorb roller 7d opposed to the adsorb corona charger, an inner corona charger 7e and an outer corona charger 7f. Further, a peripheral opening area of the transfer drum 7a rotatably supported for driven movement is covered by a cylindrical recording material bearing sheet 7g made of dielectric material.
As the transfer drum 7a is rotated, the toner images successively formed on the photosensitive drum 2 are successively transferred, by the transfer corona charger 7b, onto the recording material adsorbed and born by the recording material bearing sheet 7g. In this way, a desired number of color toner images are transferred to the recording material born by the recording material bearing sheet 7g, thereby forming a full-color image.
After the desired number of toner images were transferred in this way, the recording material is separated from the transfer drum 7a by a separation corona charger 11a and a separation pawl 11b, and then is sent to a fixing device 12. The fixing device 12 comprises a fixing roller 12a and a pressure roller 12b. The toner images on the recording material are fuzed and mixed by heat and pressure from the rollers 12a, 12b, thereby fixing the image to the recording material. In this case, offset preventing oil is coated on a surface of the fixing roller 12a so that the toner on the recording material is prevented from adhering to the surface of the fixing roller. Thereafter, the recording material to which the toner image was fixed is discharged onto a discharge tray 13.
On the other hand, after the toner images are transferred to the recording material, residual toner remaining on the photosensitive drum 2 is removed by the cleaning device 9 and residual toner remaining on the recording material bearing sheet 7g is removed by a transfer cleaner 15, thereby preparing for the next image formation.
in order to effectively use the resources, it is desirable to reduce the number of recording materials to be used, by forming images on both faces of each recording material. Accordingly, it is desirable to add a both-face image forming function to a full-color image forming apparatus. If the both-face image forming function is added to the above-mentioned full-color image forming apparatus, the following processes will be performed.
The recording material is borne by the transfer drum 7a and the toner images are formed on a first face of the recording material. Then, the recording material is sent to the fixing device 12, where the toner images are fixed to the recording material. Then, the recording material is turned over and then is borne by the transfer drum 7a again so that the toner image on the first face is contacted with the recording material bearing sheet 7g. The toner images are formed on the second face of the recording material and the toner images are fixed to the recording material by the fixing device 12.
However, when the above-mentioned processes are carried out, the offset preventing oil adhered to the recording material during the fixing of the toner images to the recording material is adhered to the recording material bearing sheet. Since the recording material bearing sheet is contacted with the photosensitive drum, the oil adhered to the recording material bearing sheet is transferred to the photosensitive drum.
Thus, when the images are formed on both first and second faces of the recording material, and particularly when a plurality of recording materials are continuously used to form images on their both faces, a great amount of oil will be adhered to the surface of the photosensitive drum 2. In this condition, if the image formation is continued, due to viscosity of the oil, there will arise a "fog" phenomenon that the toner is adhered to a blank area of the recording material which must be kept white. On the other hand, regarding solid areas of the recording material which must be made black, since the oil on the photosensitive drum 2 prevents the toner from transferring from the surface of the photosensitive drum 2 to the recording material, the toner cannot be transferred from the photosensitive drum 2 to the recording material sufficiently, thereby thinning the image.
Although the cleaner 15 is contacted with the surface of the transfer drum 7a, the cleaner is of brush type and merely serves to remove the residual toner from the transfer drum, and, thus, cannot remove the oil from the transfer drum.
In mono-color image forming apparatuses wherein a toner image is formed by single color toner, when the both-face image formation is carried out, the above-mentioned problem regarding the adhesion of oil will arise. However, in the full-color image forming apparatus wherein the full-color image is formed by superimposing a plurality of toner images having different colors, since an amount of oil adhered to the recording material is considerably great, the oil adhered to the photosensitive drum particularly affects a bad influence.